Do the mundane tasks like documentation, paperwork, emails etc

@SolarMike the problem is there's no mundane tasks to do. I'm a junior so I dont often get any emails and there's no paperwork to do. As for documentation, there's not really much of that either.

I would venture that 6 hours of straight coding is more than most people are able to pull off anyway. You're not a robot.

Are you eating well? Are you getting enough sleep? Have you tried taking a short break to see if it helps?

Can't you make 6 hours of coding appear to take 8 hours?

Why are you only taking 15-20 minutes for lunch? Writing code is highly mental. You might need to take a longer break at lunch and get away from it. I take a full hour for lunch and leave my desk on most days. I've been doing software development for almost 40 years.

I've been praised for being a high performer and received nothing but excellent reviews but my manager has noticed I tend to not get much done around those hours and wants me to do more.

"Then until 5 I'm exhausted and can't do anything." Do you suddenly regain your energy at 5? If not, when are you back in a working state?

Next day @sf02 I have energy

@softwaredev1 - have you talked to your doctor?

Is it a matter of time of day, or total time worked?

Honestly, go for a walk. Clear your mind. I used to have a job where at a certain point in the day I wandered over to a couch and sat for 10 minutes. People knew they could come and small talk or come with a big problem, but not to bother me with little stuff for a bit. It made me much more productive for the breathing room.

How old are you (rhetorical question)? If you haven't had your eyes tested recently get it done. Natural sight degradation as you age slowly creeps up on you and I found the tiredness you describe is one of the first identifiable indicators.

have you tried a siesta? works for me

Exercise, fitness, walk after lunch, afternoon espresso if it works for you, walk in the afternoon. Eat lighter lunch to avoid sugar crash in the afternoon, heavy pasta or rice in particular. Keep half of lunch or a snack. Try salads.

@Col I've never associated fatigue with natural decline in eyesight. Do you have a link?

Get promoted. You'll still be unproductive between 3pm-4pm, but you'll be unproductive in (yet another) pointless meeting where it doesn't matter so much.

@MichaelJ. No link, just personal experience. I was experiencing the issues the OP describes and after getting glasses I was back to a full day of programming. The change really was astounding.

Take 5 minute breaks every 2 hours

Programming is creative work, and creativity doesn't answer to a schedule. While you can promote it with a good environment and habits, it's still not a switch you can flip on and off. Accept that midafternoon is not a productive period for you, and listen to your body when it tells you to take a break. No one goes 100% all day in software.

Some would say (I know I would) that it won't matter how much of a high performer you are... for management it will never be enough. One common reward for exceptional work is, you guessed it, more work. A promotion? Why? Why promote a developer with an output equivalent to another three devs?

Seems like the kind of programming matters. If you're doing basic, entry-level stuff, then it's basically just mindless typing. But then other stuff might be more involved, presumably up to the limits of one's abilities.

There are very good comments above. What works for me is standing up and think about nothing or work-unrelated things for 5-10 min, regularly during the day. Your brain needs breaks along the day, it's normal. You can't be expected to be fully focused for 8 hours straight. Trying to do it might be hurtful in the long term. That's one way to get burned out. And do not feel guilty about it, everybody does it, that's how we keep our sanity.

If you have caffeine in the morning, but not the afternoon, that could be a factor. You could either have no caffeine all day or a second dose after lunch or between 1:00 and 2:00.

I have to say, a 20-30 minute nap does wonders for my programming ability in the afternoon. Easier to get away with when working from home, of course, but I'm certain that I get more done in a day that way then trying to "tough it out" and making a bunch of stupid mistakes (that I then have to go and find and fix) because my brain is exhausted.

Unless your salary is considerably more than average (for similar experience and location), you should really put minimum 2 hours per week looking for another job. Unless you are personally benefiting (ie: it's exactly what you want to be added in your resume), 12-14 hour days + weekends is unacceptable.

From my perspective my salary is less than the average junior makes and I'm doing work that probably is closer to that of a senior engineer + ops team.

Consider your caffeine consumption - as mentioned above - ALL sources of caffeine - not just coffee: most carbonated soft drinks unless specifically "non caffeine". I had the same problem. Then I swore off caffeine - and it took about 4 weeks of miserable afternoons - much worse than before! With mild headaches included! - for the withdrawal. Since then everything's been fine in the afternoon. I did it by switching from my 6+ Diet Coke's per day to 6+ DECAF Diet Coke's per day - no problem!